For individuals venturing into underwater environments, breathable gas is typically delivered from a compressed gas storage tank and demand system known as SCUBA (Self Contained Underwater Breathing Apparatus). The most common form termed open circuit, releases pressurized gas through a regulator which contains a diaphragm located adjacent the diver's mouth that senses and is responsive to the divers breathing pressure. The diaphragm acts to move a demand valve to deliver breathing gas to the diver when required and all subsequently exhaled gas typically passes back into the regulator and is directed through a one way valve into the surrounding environment where it is permanently lost for use by the diver. During inhalation, the one way valve seals the regulator off from the surrounding environment to prevent water from back flowing and choking the diver. The sensitivity of present art regulators is such that very little effort is required to inhale or exhale underwater. Since open circuit gas is used only once, divers are required to carry a large volume of pressurized gas proportional to the inhalation rate of the diver as well as to the depth the gas is breathed, which limits the amount of time a diver has available underwater to the amount of gas carried with them. As depths increase, ambient pressure increases by about 1 atmosphere for every 33 feet/10 meters. At a depth of 33 feet/10 meters, the diver is subjected to 2 atmospheres of pressure (one atmosphere at the surface plus one additional for the 33 feet/10 meters of water) and a scuba tank will last only ½ as long as it would at the surface, and at depths of 300 feet now commonly visited by divers, that same tank will last about 1/10 the duration at the surface.
As time underwater and depth continue to increase, divers increase the size and quantity of tanks until the bulk and complexity are too much to handle. To deal with this issue, divers are turning to devices known as re-breathers that capture exhaled gas in one or more flexible storage container, known commonly as a counterlung, and return a portion to the diver for re-breathing. By recycling the breathed gas, re-breathers extend usable time from a given amount of gas, by as much as 20 times. To recycle exhaled gas, the re-breather must eliminate unwanted carbon dioxide (CO2) naturally introduced by the body as part of the respiration process or death can result. To do this, re-breathers process breathing gas in a loop like fashion to pass through a device known as a CO2 scrubber containing a suitable chemical agent such as lime that chemically absorbs CO2, releasing heat and water in a well understood process. As long as the scrubber is appropriately sized and designed, CO2 free gas results that is then returned back to the diver to be breathed again, forming what is known in totality as a breathing loop.
In all presently implemented systems, re-breathed gas is driven through the loop directly by the breathing pressure of the diver and present art re-breathers require more breathing effort compared to open circuit, thus it is very important to minimize flow restrictions in the breathing loop to maintain the work of breathing (WOB) to reasonable levels. The duration and CO2 removal capability of the scrubber is increased by employing larger, more complex scrubbers filled with agent made with finer sized granules that all act together to increase the total available active surface area, and unfortunately, also act to increase the WOB required in the loop. Work of breathing and scrubber duration are two of the most important performance criteria for re-breathers since it is critical to the survival of the diver that they be able to breathe easily enough to adequately ventilate the body with enough metabolic oxygen to survive as well as expel CO2 that is metabolically produced by the body. Once outside the body, CO2 must be continually and effectively removed from the breathing loop by the scrubber or an incapacitating condition known as hypercapnia can result which, although it might be survivable at the surface, can easily lead to death while underwater. It is particularly desirable to reduce the WOB of the entire rebreathing system and add safety features that would otherwise not be practical due to the associated increase in WOB such as an improved CO2 scrubber.
One idea to reduce WOB that several divers have considered and typically rejected for reasons of complexity, uses compressed gas carried by the diver to assist the counterlung in expansion and contraction during breathing such as described in U.S. published patent application No. 2001/0015203. This approach, teaches about a device that adds pressurized drive gas to a small isolated sub-section of the flexible storage container that collects exhaled breathing gas from the diver, where the added drive gas acts to assist the diver in the exhalation process. Inhalation is also assisted by bleeding off the added pressurized drive gas to allow the flexible storage container to forcibly contract. The device trades work by the diver for work by the drive gas acting on the flexible storage container to move breathing gas through the loop resulting in a reduction in WOB by the diver. The extra motive force created by the drive gas acting on the flexible storage container also creates higher pressures in the breathing loop rather than in the diver's lungs. With each breathe, the volume of added drive gas builds up and must be vented from the loop or an over pressurize condition will result. The device features one way pressure relief valves to rid the breathing loop of this excess breathing gas and exhaust it to the surrounding environment. Ideally, this exhaust would occur once the flexible storage container is fully expanded and cannot contain additional gas. In this case assisted breathing no longer will function and the diver must create the motive force to expel the excess gas. Accordingly, these one way valves must be set to open at a pressure low enough that allows the diver to comfortably expel excess breathing gas without assistance when the loop is filled to full capacity. Unfortunately, the higher loop pressures that occur during assisted breathing causes breathing gas to undesirably escape from the loop through the one way pressure relief valves set to relieve at these lower pressures and extensive testing has shown that so much breathing gas is lost as to render the concept useless. It would be extremely desirable to provide an assisted re-breathing device that does not prematurely leak breathing gas to the surrounding environment during assisted breathing which also provides for unassisted exhalation of excess breathing gas from the loop to the surrounding environment by the diver with a lower breathing pressure similar to open circuit scuba.
In prior art re-breathers, additional one way valves are used to ensure un-scrubbed exhale gas laden with CO2 is not re-breathed and is instead directed to pass properly through the scrubber. These valves are typically located as close to the mouth as possible to minimize the volume of gas that can be directly re-inhaled, one positioned to allow exhaled gas to pass down into the re-breather for temporary storage and scrubbing and a separate one turned in the opposite direction to receive scrubbed gas back from the re-breather and pass it back to the diver for inhalation. Most one way valves are designed as simple flexible membranes that under reverse flow conditions, normally act to effectively seal off flow passages and open only under forward flow conditions to allow flow to move in the proper direction around the loop. These one way valves resist the flow of breathing gas and add to the WOB, increasing resistance with increased breathing gas flow. To reduce WOB, a minimum number of one way valves are used and their size is maximized. Many accidents have been reported involving the failure of one way valves that allowed exhaled gas to be directly re-inhaled, leading to buildup of CO2 in the loop. It is highly desired to maintain loop flow direction integrity when failure of a one way check valve occurs.
Particularly insidious is that hypercapnia can arise quite rapidly. A condition commonly known as breakthrough occurs when the scrubbing agent is depleted in any location enough to allow a significant portion of CO2 to pass through the scrubber, rendering it unusable. It is well understood that heavy breathing and/or deeper depths cause CO2 to pass further through the scrubber which can lead to early breakthrough. Breakthrough can also occur due to improper packing of the agent into the scrubber with a condition known as channeling, where re-breathed gas follows a low resistance to gas flow path that quickly depletes the locally surrounding scrubbing agent and allows CO2 to prematurely channel through the scrubbing bed. Warning systems for the presence of CO2 have only recently been introduced with limited success due to extreme sensitivity exhibited by available sensors to high relative humidity environments such as what exists naturally in a re-breather loop. Prior art systems that do exist, employ barriers made of sponges and/or water impermeable membranes placed between the loop and the sensor to limit water intrusion, which unfortunately also degrades the response time of the sensor, making it relatively ineffective when CO2 rapidly builds up, or worse, can render the sensor useless if water saturation of the barrier occurs. Instead, most divers today rely on indirect measurements such as the time a scrubbing agent bed has been in service versus conservative experience as well as the direct measurement of scrubber temperature to determine when to stop using the bed. Since the scrubbing process naturally generates heat, a temperature rise indicates the agent is being activated by the presence of CO2 and when this occurs near the end of the bed, it is time to stop using the re-breather. Unfortunately, CO2 breakthrough tends to occur quite suddenly, especially during periods of heavy exertion and/or at deeper depths, making predictions based on time and temperature quite fallible. Due to significant safety concerns, re-breather divers would like to quickly and reliably monitor for the life threatening presence of CO2 in the loop, especially at higher work levels and/or deeper depths when a rapid buildup can occur without warning.
Re-breathers also must provide make up for oxygen absorbed from inhaled gas by the body to satisfy the metabolic needs of the diver. It is well understood that at constant workload, the rate of metabolic oxygen consumption is more or less constant, requiring roughly the same number of O2 molecules per unit time, meaning oxygen consumption is proportional to mass flow. Additionally, it is well known that metabolism and associated oxygen consumption by the divers body, changes more or less proportional to workload and respiration rate, therefore the harder you work, the higher the respiration rate, and the larger the mass flow requirement for metabolic makeup oxygen. Similar to open circuit gas consumed with depth, if a fixed mass sample of oxygen were isolated in a flexible container at the surface, it would shrink to ½ the volume at 33 feet and 1/10th the volume at 300 feet, yet this fixed mass would sustain the diver for the same period of time metabolically. Overall, metabolic consumption requires makeup oxygen volumetric flow to increase proportional to respiration rate and drop inversely proportional to depth induced pressure.
In diving operations, the amount of oxygen present in the breathing gas is measured in terms of oxygen partial pressure or PO2, usually expressed in standard atmospheres of pressure. Normal oxygen at the surface is 21% of one atmosphere and is expressed as 0.21 PO2, whereas 100% pure oxygen at the surface is 1.00 PO2. It is well recognized that for safe diving operations, the oxygen content of breathing gas should always remain in the range of about 0.16<PO2<1.60. Too little oxygen, known as hypoxia, is a deadly condition that occurs below around 0.16 PO2, where insufficient oxygen is present to sustain life. Too much oxygen, termed hyperoxia, becomes toxic over time to the central nervous system (CNS). Commonly referred to by divers as CNS toxicity, high oxygen levels eventually lead to uncontrolled convulsions, which when convulsions occur underwater, place the diver at extreme risk of death due to drowning. Typically this condition strikes without warning, and evidence supports that toxicity is accelerated by elevated CO2 levels. Several prior art methods are used in re-breathers, that attempt to maintain safe levels of oxygen in the breathing loop.
One method, such as employed in U.S. Pat. No. 6,526,971 and present art rebreathers known as the RB80, forcibly eject a portion of the exhaled gas from the loop that is replaced by passive addition of a gas mix containing some amount of oxygen, linked to the respiration rate of the diver. Oxygen levels in the loop drop and stabilize to several percent below injected levels. Divers must be very careful not to allow hypoxia to set in, especially at shallower depths where the percent drop is amplified. Another method allows the diver to manually actuate a valve to add oxygen to the loop as required. Yet another method bleeds oxygen into the loop using a fixed orifice driven by a special regulator designed to maintain a constant, absolute pressure on one side of the orifice, with ambient pressure on the other, with enough differential pressure to cause sonic flow through the orifice. As ambient pressure increases with depth, pressure across the orifice drops, producing a roughly constant mass flow of oxygen as depth changes in a well understood process that is not linked to the respiration rate of the diver. As ambient pressure increases sufficiently to cause the orifice to drop into sub-sonic operation, mass flow is reduced, ultimately to zero when ambient pressure equals the set pressure of the absolute pressure regulator. In these systems, the diver normally chooses an orifice sized to produce flow somewhat below their resting metabolic rate such that an occasional manual add of oxygen is required to make up for any shortfall and more frequent additions are required at deeper depths when the orifice goes sub-sonic and with increased workloads. It is very important in these systems that the diver closely monitor oxygen content within the breathing loop so that timely additions can be made to remain safe. Another system automatically monitors and controls oxygen addition using an electronic closed loop computer control system that periodically cycles an electric oxygen addition valve to maintain oxygen levels within the loop. Electronic systems are susceptible to failure in underwater environments and it is very important that oxygen monitoring sensors be accurate to facilitate safe operation of re-breathers employing them.
Prior art oxygen monitoring within the re-breathing loop is commonly accomplished using some form of galvanic sensor which unfortunately, are proven in practice to not be all that reliable. Sensors typically exhibit a relatively short life expectancy of just several months to a year or two, are also susceptible to malfunction when exposed to condensing water and provide little warning they are about to fail. When they do wear out or fail, they report oxygen levels different from what is actually present. Due to reliability concerns, divers typically employ multiple sensors, sequence them in age, and even employ sophisticated real time computer algorithms to determine the health and believability of sensors. In the end, sensor health is left up to the diver to evaluate and this requires constant vigilance to remain safe. It is particularly desirable to eliminate the need for electronic controls and sensor feedback to properly add and maintain oxygen levels in underwater re-breathing devices.